National Radio Astronomy Observatory

Charlottesville, Virginia

Challenges for FASR:

• Extremely wide bandwidth (100 MHz to 30 GHz)

• Small size of the antenna (diameter of 3 – 5 meters)

• Dual polarization

• “Acceptable” G/T

• Minimize number of moving parts

Gain of Reflector Antennas

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B

X

P

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Gain of Reflector Antennas

1X =ηFeed pattern is symmetric (E and H plane patterns are identical )

1P =ηCommon phase center for both planes

Illumination and spillover losses are the principal causes of gain degradation.

There is usually a compromise between the illumination and spillover.

sidelobes. increasedin results Thisfeed. realizable afor low toois then 1, If SI ηη =

Clarricoats, P.J.B. and A.D. Olver, Corrugated Horns for Microwave Antennas, Peter Peregrinus, Ltd., London, UK, 1984, p. 100.

Types of Feeds Presented Here

• Hybrid-Mode

• Dipole Above Ground Plane

• Frequency Independent Structures

Hybrid-Mode Feeds

The Hybrid-Mode Feed offers a means to approach all of the objectives simultaneously.

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=

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HE

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Clarricoats, P.J.B. and A.D. Olver, Corrugated Horns for Microwave Antennas, Peter Peregrinus, Ltd., London, UK, 1984, p. 7.

Hybrid-Mode Feed – Corrugated Waveguide

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Co-polar bandwidth

Position of the phase center and generation of higher order modes Pattern symmetry and

cross-polar characteristics

Impedance matching to the waveguide and mode conversion level Clarricoats, P.J.B. and A.D. Olver, Corrugated Horns for

Microwave Antennas, Peter Peregrinus, Ltd., London, UK, 1984, pp. 99-100.

Hybrid-Mode Feed – Corrugated Waveguide

Large flare angle horn (scalar feed) gives a relatively large bandwidth with nearly constant co-polar radiation characteristics.

Thomas, James, and Greene (1986):

Scalar feed having 2.1:1 bandwidth.

James (1984):

Compact profiled horn having 2.4:1 bandwidth (requires some focus adjustment)

Clarricoats, P.J.B. and A.D. Olver, Corrugated Horns for Microwave Antennas, Peter Peregrinus, Ltd., London, UK, 1984, p. 21.

Hybrid-Mode Feed - Corrugated Waveguide

Absorber Lined Horn

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(1986) Ostertag and Cheng, Knop,

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Knop, C.M., Y.B. Cheng, and E.L. Ostertag, “On the Fields in a Conical Horn Having an Arbitrary Wall Impedance,” IEEE Trans. Ant. and Propag., Vol. 34, Sept. 1986, pp. 1092-1098.

Hybrid-Mode Feed – Surface-Wave Antenna

Dielectric Waveguide (rod)

( ) ( )( )

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nn

Yn

∆−=−

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=∆

=

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2)(

Thus,

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thenunity, from n, amount, small aonly by differsindex refractive theif and 1kr If 1

This condition is frequency independent if the dielectric is free from dispersion over the operational bandwidth.

Unless delta-n is sufficiently large it is impossible to effectively launch the dominant mode of the dielectric waveguide – loose energy in the launcher.

Corrugated waveguide: Lower cross-pol over 2.5:1 bandwidth.

Dielectric waveguide: higher cross-pol over a much wider band.BW= 4:1

Clarricoats, P.J.B. and A.D. Olver, Corrugated Horns for Microwave Antennas, Peter Peregrinus, Ltd., London, UK, 1984, p. 9.

Johnson, R. C. Antenna Engineering Handbook,, 3rd

ed., McGraw-Hill, New York, 1993, p. (12-21).

Hybrid-Mode Feeds

Disadvantages for FASR:

• Big and bulky (corrugated)

• High machining cost (corrugated)

• Bandwidth not large enough

• Polarizer bandwidth about 2.5:1

• Loss associated with dielectric material

Dipole Above a Ground PlaneConventional half-wave dipole

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (4-12).

Dipole Above a Ground PlaneConventional half-wave dipole

BW=1.09:1Stutzman, W. and G. Thiele, Antenna Theory and Design, Wiley, New York, 1981, pp. 199-200.

Dipole Above a Ground PlaneFat Dipole

BW= 1.18:1Stutzman, W. and G. Thiele, Antenna Theory and Design, Wiley, New York, 1981, p. 203.

Dipole Above a Ground Plane

Sleeve Dipole

BW= 1.67:1Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (4-21).

Dipole Above a Ground Plane

Folded, Fat Dipole

BW= 1.53:1

Dipole Above a Ground Plane

Short Backfire Antenna

Ehrenspeck, H.W. “The Short Backfire Antenna,” Proc. of the IEEE, v. 53, Aug. 1965, pp.1138-1140.

BW= 1.35:1Srikanth, S. and G. Behrens, “A New Short Backfore Antenna as a Prome Focus Feed for the GBT”, GBT Memo. #187, May 13, 1998.

Dipole Above a Ground Plane

Disadvantages for FASR:

• Bulky

• Narrow bandwidth

• Large spillover

Frequency Independent Antennas

Provided the antenna is made of practically perfect conductors and dielectrics, the impedance, polarization, pattern, etc. are invariant to a change in scalethat is in proportion to the change in wavelength.

If the shape of the antenna is determined entirely by angles (invariant to scale), the performance would have to be independent of frequency.

Truncation of the shape from that of infinite extent should also give goodperformance (current should decrease with distance from the terminals).

Ω===⇒ 5.1882

Designary Complement-Self ηMETALAIR ZZ

Operational bandwidth:

• lowest frequency limited by the maximum dimension

• upper frequency limited by the input terminal region

• bandwidths can be 10:1 or greater.

Frequency Independent Antennas

Log Periodic

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-30).

Frequency Independent Antennas

Spirals

Stutzman, W. and G. Thiele, Antenna Theory and Design, Wiley, New York, 1981, p. 284.

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-25).

Frequency Independent Antennas

Absorber Loaded Cavity

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-18).

Frequency Independent Antennas

Conical Log-Spiral

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-33).

Frequency Independent Antennas

Sinuous

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-56).

Frequency Independent Antennas

Sinuous

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-60).

Frequency Independent Antennas

Sinuous

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-61).

Frequency Independent Antennas

Sinuous Antenna on a Cone

Johnson, R. C. Antenna Engineering Handbook,, 3rd ed., McGraw-Hill, New York, 1993, p. (14-61).

Frequency Independent Antennas

Disadvantages for FASR:

• Integration of feed, LNA and balun

• Could be lossy (affect G/T)

• Dual-polarization

• Gain varies with frequency for fixed position conical

Conclusions

For FASR:

• Frequency independent antennas are the only practical option to cover FASR frequency range.

• Use a cavity backed structure for wide instantaneous bandwidth.

• Accept gain reduction associated with fixed position cone.

• Consider sinuous version of frequency independent structures.

• Integration of feed terminals with LNAs and balun.

• Prototyping highly recommended!